The long-term objective is to determine the role of respiratory tract cytochrome P450 (P450) enzymes in target tissue metabolic activation and toxicity of environmental chemicals. The current focus is on a recently identified human P450 enzyme, CYP2A13. Preliminary studies indicated that (a) CYP2A13 mRNA is expressed mainly in the respiratory tract and is much more abundant than CYP2A6, the other functional member of the CYP2A subfamily, in nasal mucosa and lung; (b) heterologously expressed CYP2A13 is highly active in the metabolic activation of several compounds known or suspected to cause nasal and lung cancers in experimental animals and in humans, such as N-nitrosodiethylamine (NDEA) and 4-(methylnitrosamino)-l-(3-pyridyl)-l-butanone (NNK); and (c) CYP2A proteins are expressed in human fetal nasal mucosa at levels much higher than in fetal liver. We thus hypothesize that CYP2A13 provides a unique metabolic activation pathway in human nasal and lung tissues to initiate chemical carcinogenesis and that this pathway is already active during fetal development. Two specific aims (Aims 1 and 2) are designed to fully characterize this new human P450 enzyme with respect to its expression, its contribution to metabolic activation of a known respiratory tract procarcinogen (NNK) in human fetal nasal and pulmonary microsomes, and the extent and functional consequences of its genetic polymorphisms. In addition (Aim 3), a novel, lung-specific NADPH-cytochrome P450 reductase (CPR) knockout mouse model will be used to test a related hypothesis that pulmonary P450s are responsible for NNK metabolic activation and NNK-induced lung tumors. The proposed studies will fill an important knowledge gap regarding human enzymes responsible for tobacco-related chemical carcinogenesis in the respiratory tract, and will contribute toward a more accurate prediction of individual risks of toxicity from environmental chemical exposure in fetuses and well as in adults. They will also help to improve our understanding of the genetic factors involved in the diseases of the respiratory tract, such as lung cancer, which is the leading cause of cancer-related death in the U.S.